The ATSC digital television standard has been extended to allow a field to contain a mix of more robustly coded data (referred to herein as enhanced data) and the data currently provided for in the standard (referred to herein as VSB data). Further similar extensions are being investigated. Preferably, though not necessarily, the data mix is employed on a segment-by-segment basis such that, ignoring the effects of interleaving in the encoder, some segments of a field are used to transmit VSB data exclusively and the remaining segments of the field are used to transmit enhanced data segments exclusively. However, it is possible that all data segments of a field could contain either enhanced data segments exclusively or VSB data segments exclusively. Moreover, it is also possible that some of the enhanced data contained in a field may be coded with one robust coding rate and that other enhanced data contained in the field may be coded at other robust coding rates.
Accordingly, the ATSC digital television standard permits the transmission of enhanced data packets along with the typical main signal packets such as video packets, audio packets, and control packets usually associated with digital television programming. As indicated above, the data in the enhanced data packets is enhanced with extra coding in order to provide greater robustness so as to increase the likelihood that the enhanced data will be properly received in spite of noise, such as noise caused by the transmission channel.
In many transmission schemes for the transmission of enhanced data, the introduction of the enhanced data packets forces the main signal packets to new time locations in the transport stream. This relocation of the main signal packets is commonly called packet jitter.
The term “legacy receiver” is used herein to denote older digital television receivers that have not been built to effectively receive and process a digital television signal containing enhanced data packets. The buffers, particularly the audio buffers, of legacy receivers tend to be too small to permit these legacy receivers to effectively manage a received digital television signal containing enhanced data packets. As a result, the transmission of a digital television signal containing enhanced data causes packet jitter in legacy receivers.
For example, according to ISO 13818-1, the decoder of a receiver includes a transport buffer definition that limits the maximum data rate. Similarly, a main buffer holds a larger portion of data to be processed and essentially defines a minimum data rate burst for a particular type of transport stream. The maximum data rate translates to multiple packet advancement in time, and the minimum data rate translates to packet delay.
A single packet displacement is easily handled by the buffers. However, displacement of larger groups of packets becomes problematic. Therefore, because of the buffer limitations, the packets in the transport stream must be carefully ordered in order to reduce packet jitter.
The present invention overcomes one or more of these or other problems.